Yes, that is the manner in which Sync works; selected data types per device are synchronized on both directions to & from the Sync server when a 'new device' is connected to Sync. Then when other devices open Firefox the synchronization process continues to 'that device'. Other than selected data types and on/off there are no "user controls" with Sync. The '''undo''' for Sync for a particular device was removed from Firefox a few versions in the past; evidently it was a temporarily added feature for Sync ''(maybe for 'testing' and was enabled for just a few versions)'' and I guess that "it didn't cut the mustard", so it got axed due to complaints and at least one Bugzilla report. There is no "reset" for user saved data beyond creating a new Profile and starting from scratch. The "Refresh" procedure would save specific user data (bookmarks and logins data) to a "refreshed Profile". All user customizations would be gone. As far as bookmark backups goes, Firefox does save 15 backups of the users bookmarks (only); restoring a bookmarkbackup file would delete saved browsing history, AFAIK. ''That's not something I have ever had the need to do; restoring a bookmarksbackup.'' If that interests you, see this KB article: https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them